In order to manage threatening situations, humans have evolved to experience a "fight or flight" response. As part of this response, when humans are confronted with a dangerous situation, their body mobilizes by sending blood away from their extremities (e.g. hands and feet) and into the major muscles, producing adrenaline, and increasing heart rate so that we are better equipped to fight off danger.
Occasional anxiety is an expected part of life. You might feel anxious when faced with a problem at work, before taking a test, or before making an important decision. But anxiety disorders involve more than temporary worry or fear. For a person with an anxiety disorder, the anxiety does not go away and can get worse over time. The symptoms can interfere with daily activities such as job performance, school work, and relationships.